Neurodegenerative diseases, such as glaucoma, macular degeneration, and Alzheimer's disease, affect millions of patients throughout the world. Because the loss of quality of life associated with these diseases is considerable, drug research and development in this area is of great importance.
Age-related macular degeneration (AMD) affects between ten and fifteen million patients in the United States, and it is the leading cause of blindness in aging populations worldwide. AMD affects central vision and causes the loss of photoreceptor cells in the central part of retina called the macula. Macular degeneration can be classified into two types: dry-form and wet-form. The dry-form is more common than the wet; about 90% of age-related macular degeneration patients are diagnosed with the dry-form. The wet-form of the disease and geographic atrophy, which is the end-stage phenotype of dry-form AMD, causes the most serious vision loss. All patients who develop wet-form AMD are believed to previously have developed dry-form AMD for a prolonged period of time. The exact causes of AMD are still unknown. The dry-form of AMD may result from the senescence and thinning of macular tissues associated with the deposition of pigment in the macular retinal pigment epithelium. In wet-form AMD, new blood vessels grow beneath the retina, form scar tissue, bleed, and leak fluid. The overlying retina can be severely damaged, creating “blind” areas in the central vision.
For the vast majority of patients who have the dry-form of AMD, no effective treatment is yet available. Because the dry-form of AMD precedes development of the wet-form of AMD, therapeutic intervention to prevent or delay disease progression in the dry-form AMD would benefit patients with dry-form of AMD and might reduce the incidence of the wet-form of AMD.
Decline of vision noticed by the patient or characteristic features detected by an ophthalmologist during a routine eye exam may be the first indicator of AMD. The formation of “drusen,” or membranous debris beneath the retinal pigment epithelium of the macula is often the first physical sign that AMD is developing. Late symptoms include the perceived distortion of straight lines and, in advanced cases, a dark, blurry area or area with absent vision appears in the center of vision; and/or there may be color perception changes.
Different forms of genetically-linked macular degenerations may also occur in younger patients. In other maculopathies, factors in the disease are heredity, nutritional, traumatic, infection, or other ecologic factors.
Glaucoma is a broad term used to describe a group of diseases that causes a slowly progressive visual field loss, usually asymptomatically. The lack of symptoms may lead to a delayed diagnosis of glaucoma until the terminal stages of the disease. The prevalence of glaucoma is estimated to be 2.2 million in the United States, with about 120,000 cases of blindness attributable to the condition. The disease is particularly prevalent in Japan, which has four million reported cases. In many parts of the world, treatment is less accessible than in the United States and Japan, thus glaucoma ranks as a leading cause of blindness worldwide. Even if subjects afflicted with glaucoma do not become blind, their vision is often severely impaired.
The progressive loss of peripheral visual field in glaucoma is caused by the death of ganglion cells in the retina. Ganglion cells are a specific type of projection neuron that connects the eye to the brain. Glaucoma is usually accompanied by an increase in intraocular pressure. Current treatment includes use of drugs that lower the intraocular pressure; however, contemporary methods to lower the intraocular pressure are often insufficient to completely stop disease progression. Ganglion cells are believed to be susceptible to pressure and may suffer permanent degeneration prior to the lowering of intraocular pressure. An increasing number of cases of normal-tension glaucoma are observed in which ganglion cells degenerate without an observed increase in the intraocular pressure. Current glaucoma drugs only treat intraocular pressure and are ineffective in preventing or reversing the degeneration of ganglion cells.
Recent reports suggest that glaucoma is a neurodegenerative disease, similar to Alzheimer's disease and Parkinson's disease in the brain, except that it specifically affects retinal neurons. The retinal neurons of the eye originate from diencephalon neurons of the brain. Though retinal neurons are often mistakenly thought not to be part of the brain, retinal cells are key components of the central nervous system, interpreting the signals from the light-sensing cells.
Alzheimer's disease (AD) is the most common form of dementia among the elderly. Dementia is a brain disorder that seriously affects a person's ability to carry out daily activities. Alzheimer's is a disease that affects four million people in the United States alone. It is characterized by a loss of nerve cells in areas of the brain that are vital to memory and other mental functions. Currently available drugs can ameliorate AD symptoms for a relatively finite period of time, but no drugs are available that treat the disease or completely stop the progressive decline in mental function. Recent research suggests that glial cells that support the neurons or nerve cells may have defects in AD sufferers, but the cause of AD remains unknown. Individuals with AD seem to have a higher incidence of glaucoma and age-related macular degeneration, indicating that similar pathogenesis may underlie these neurodegenerative diseases of the eye and brain. (See Giasson et al., Free Radic. Biol. Med. 32:1264-75 (2002); Johnson et al., Proc. Natl. Acad. Sci. USA 99:11830-35 (2002); Dentchev et al., Mol. Vis. 9:184-90 (2003)).
Neuronal cell death underlies the pathology of these diseases. Unfortunately, very few compositions and methods that enhance retinal neuronal cell survival, particularly photoreceptor cell survival, have been discovered. A need therefore exists to identify and develop compositions that can be used for treatment and prophylaxis of a number of retinal diseases and disorders that have neuronal cell death as a primary, or associated, element in their pathogenesis.
In vertebrate photoreceptor cells, the irradiance of a photon causes isomerization of 11-cis-retinylidene chromophore to all-trans-retinylidene and uncoupling from the visual opsin receptors. This photoisomerization triggers conformational changes of opsins, which, in turn, initiate the biochemical chain of reactions termed phototransduction (Filipek et al., Annu. Rev. Physiol. 65:851-79 (2003)). Regeneration of the visual pigments requires that the chromophore be converted back to the 11-cis-configuration in the processes collectively called the retinoid (visual) cycle (see, e.g., McBee et al., Prog. Retin. Eye Res. 20:469-52 (2001)). First, the chromophore is released from the opsin and reduced in the photoreceptor by retinol dehydrogenases. The product, all-trans-retinol, is trapped in the adjacent retinal pigment epithelium (RPE) in the form of insoluble fatty acid esters in subcellular structures known as retinosomes (Imanishi et al., J. Cell Biol. 164:373-87 (2004)).
In Stargardt's disease (Allikmets et al., Nat. Genet. 15:236-46 (1997)), a disease associated with mutations in the ABCR transporter that acts as a flippase, the accumulation of all-trans-retinal may be responsible for the formation of a lipofuscin pigment, A2E, which is toxic towards retinal pigment epithelial cells and causes progressive retinal degeneration and, consequently, loss of vision (Mata et al., Proc. Natl. Acad. Sci. USA 97:7154-59 (2000); Weng et al., Cell 98:13-23 (1999)). Treating patients with an inhibitor of retinol dehydrogenases, 13-cis-RA (Isotretinoin, Accutane®, Roche), has been considered as a therapy that might prevent or slow the formation of A2E and might have protective properties to maintain normal vision (Radu et al., Proc. Natl. Acad. Sci. USA 100:4742-47 (2003)). 13-cis-RA has been used to slow the synthesis of 1 l-cis-retinal by inhibiting 11-cis-RDH (Law et al., Biochem. Biophys. Res. Commun. 161:825-9 (1989)), but its use can also be associated with significant night blindness. Others have proposed that 13-cis-RA works to prevent chromophore regeneration by binding RPE65, a protein essential for the isomerization process in the eye (Gollapalli et al., Proc. Natl. Acad. Sci. USA 101:10030-35 (2004)). Gollapalli et al. reported that 13-cis-RA blocked the formation of A2E and suggested that this treatment may inhibit lipofuscin accumulation and, thus, delay either the onset of visual loss in Stargardt's disease or age-related macular degeneration, which are both associated with retinal pigment-associated lipofuscin accumulation. However, blocking the retinoid cycle and forming unliganded opsin may result in more severe consequences and worsening of the patient's prognosis (see, e.g., Van Hooser et al., J. Biol. Chem. 277:19173-82 (2002); Woodruff et al., Nat. Genet. 35:158-164 (2003)). Failure of the chromophore to form may lead to progressive retinal degeneration and may produce a phenotype similar to Leber Congenital Amaurosis (LCA), which is a very rare genetic condition affecting children shortly after birth.